1. Field of the Invention
The present invention relates to an image reading apparatus which reads image information of an original by forming an image representative of the image information on an image pickup element through an image forming lens, and to an adjusting method for the image reading apparatus.
2. Description of Related Art
Heretofore, there is a known image reading apparatus, as shown in FIG. 5, which reads image information of an original or the like with a solid-state image pickup element (CCD sensor) by scanning the original by every line in a sub-scan direction.
Referring to FIG. 5, an original 2 placed onto an original-placing glass board 1 is illuminated with an illuminating light source 3, such as a halogen lamp, and reflectors 4, 5 and 6. Information light reflected from the original 2 is guided to an image forming lens 11 via a slit 7a of a slit plate 7, first to third reflecting mirrors 8, 9 and 10, and is then imaged to form an image on a solid-state image pickup element 12. The image information is thus read. While the original 2 is scanned in the sub-scan direction, the scanning speeds of the mirrors 8, 9 and 10 are set such that, letting the speed of the mirror 8 be denoted by v, the speeds of the mirrors 9 and 10 are set to be v/2.
The image information obtained on the solid-state image pickup element 12 is converted into an electrical signal. The electrical signal may be outputted to an output apparatus (not shown) as the image information to be printed, or is outputted to a storage device as the image information to be stored therein. In each case, the image reading apparatus is employed.
In the image reading apparatus having the above construction, there are two methods which are conventionally available to adjust an optical system (the reflecting mirrors, the image forming lens and the solid-state image pickup element).
The first one of the two adjusting methods is explained by reference to FIG. 6(a) and FIG. 6(b). FIG. 6(a) is a cross-sectional view showing the body of the image reading apparatus, and FIG. 6(b) shows the details of a unit F included in the image reading apparatus body shown in FIG. 6(a). Units required to be adjusted in the image reading apparatus body are three units D, E and F. Referring to FIG. 6(a), the unit D includes the first mirror 8 and a lighting system (the illuminating light source 3, reflectors 4, 5 and 6, and slit plate 7), the unit E includes the second mirror 9 and the third mirror 10, and the unit F includes the image forming lens 11 and the solid-state image pickup element 12.
Main adjustment items for the optical system in the image reading apparatus body are as follows.
(1) Magnification PA1 (2) Scanning synchronization in sub-scan direction PA1 (3) Scan centering PA1 (4) Perpendicularity to sub-scan direction PA1 (5) Focus PA1 (1) Position Q of the image forming lens 11 in the direction of focus PA1 (2) Position P of the unit G in the direction of focus PA1 (3) Angular position S of the unit G around Z-axis PA1 (4) Position O of the unit H in the Z-axis direction PA1 (5) Angular position K of the unit H around X-axis PA1 (6) Position R of the unit H in the Y-axis direction
Referring to FIG. 6(b), the unit F is composed of a sub-unit F-1 including the image forming lens 11 and a sub-unit F-2 including the solid-state image pickup element 12. The unit F is subjected to an adjustment in a separate step prior to the assembling of the unit F onto the apparatus body so as to adjust the positional relationship between the sub-unit F-1 and the sub-unit F-2. Specifically, adjustment is performed, with respect to a mounting plate 13 of the image forming lens 11 in the sub-unit F-1, for the amounts of shift of the subunit F-2 in the directions parallel with X-axis, Y-axis and Z-axis and for the angles of rotation of the sub-unit F-2 around X-axis and Z-axis. Such adjustment is performed using tools without mirrors being involved.
After the adjustment, the sub-unit F-1 and the sub-unit F-2 are fixed-to each other through the mounting plate 13 and the like, using screws or solder. The unit F, after the adjustment, is assembled into the apparatus body and serves thereafter as a positioning reference in the apparatus body. The unit F is adjusted in the direction of height (in the direction of O in FIG. 6(a)) with respect to a guide rail (not shown) that serves as a reference for the first to third mirrors 8, 9 and 10 during the scanning operation, and is then fixed to the apparatus body. Furthermore, the unit F is adjusted in the direction of P to adjust magnification in the apparatus by compensating for an error in the optical path length.
Further, the unit D is adjusted in terms of the synchronization in scanning (the reading position in the sub-scan direction). The unit E is adjusted in the vertical direction L so that the reading position is adjusted in the direction perpendicular to a scan area.
In the first adjusting method, as discussed above, the image reading apparatus body has three main divided units D, E and F to be adjusted. If the unit F is found to be faulty after being assembled, it is replaced with a good unit without any adjustment involved. Specifically, the unit F is positioned and fixed with respect to a reference pin (not shown) adjustably movable on the apparatus body so that the unit F compensates for errors in the apparatus body (in the optical path length and the height referenced to the guide rail) and re-takes a preceding position.
The second adjusting method needs no mirror adjustment on an image reading apparatus body, as shown in FIGS. 7(a) and 7(b). FIG. 7(a) is a cross-sectional view showing the image reading apparatus body, and FIG. 7(b) is a detailed view of units G and H included in the apparatus body. To be adjusted in the apparatus body are the unit G including an image forming lens 11 and the unit H including a solid-state image pickup element 12. The unit H needs to be adjusted in a separate step prior to its assembly into the apparatus body. In this step, the solid-state image pickup element 12, serving as a reference for the unit H, is adjusted in the direction of focus, denoted by P, relative to a mounting plate 14. This adjustment involves the movement of the unit H in parallel with X-axis and the rotation of the unit H around Z-axis. The reason why the unit H needs adjusting in the separate step is that there is a possibility that the unit H is entirely replaced after being assembled into the apparatus body.
The units G and H, assembled in the apparatus body, are adjusted in the following items.
Among the above items, the items (1) and (2) are for adjusting magnification and focus by compensating for the error in the optical path length in the apparatus body. The item 3 is for compensating for an error in the image quality of the image forming lens 11. The item 4 is for adjusting the scanning synchronization in the sub-scan direction, the item 5 is for adjusting the perpendicularity to the sub-scan direction, and the item 6 is for adjusting the scan centering.
However, in the above first adjusting method, since each of the three units D, E and F needs to be individually adjusted in the image reading apparatus body, the adjustment step is time consuming, and each unit, because of its own particular adjusting mechanism, is mechanically complex in structure.
Further, in the above second adjusting method, the number of the units to be adjusted in the image reading apparatus body is two, i.e., only two units G and H, thus being smaller than that in the first adjusting method. However, each unit in the second adjusting method has a larger number of adjusting points, and the number of overall adjustments in the apparatus body is larger. In addition, since each unit is adjusted in the apparatus body, an adjusting mechanism has to be built in each unit. The adjusting mechanism remains continuously inside the apparatus after adjustment, occupying an internal space of the apparatus body, and thereby making the apparatus body bulky and costly.